Rewards are unlocked at certain savings milestones.
You can collect your rewards by filling up the milestone circle on the home screen. At this point the circle will glow red, meaning you've successfully hit the saving milestone and you're ready to be rewarded! Simply tap the milestone circle to collect your reward. 🎉
Note that you also need to unlock your reward after collecting it. This can be done by switching to a Yomo Investing Level, which will be available in Summer 2018.